The present invention relates to a refrigerating oil composition for a refrigerator using a carbon dioxide (CO2) refrigerant. The refrigerating oil composition can be used for compression-type refrigerators using a carbon dioxide refrigerant which are, specifically, refrigerating and air conditioning apparatuses such as automobile air conditioners, refrigerators, freezers, air conditioners and heat pumps.
In general, refrigerating cycle of compression-type refrigerators constituted with a compressor, a condenser, an expansion valve and an evaporator has a structure in which a mixed fluid of a refrigerant and a lubricating oil is circulated in the closed system. Heretofore, chlorofluorocarbons such as dichlorodifluoromethane (R-12) and chlorodifluoromethane (R-22) have been used as the refrigerant and various types of lubricating oils have been produced and used in combination with the refrigerant. However, since there is the possibility that the chlorofluorocarbons cause environmental pollution such as ozonosphere destruction in stratosphere when these substances are released into the atmosphere, the regulation on the chlorofluorocarbons is becoming stricter worldwide. Due to this situation, novel refrigerants such as hydrofluorocarbons and fluorocarbons, typical examples of which include 1,1,1,2-tetrafluoroethane (R-134a), are attracting attention. However, there is the possibility that the hydrofluorocarbons and the fluorocarbons might cause global warming due to the long life of these substances in the atmosphere although there is no possibility that these substances cause the above environmental pollution. Therefore, the use of a refrigerant derived from a natural substance which does not cause the above problems is considered.
Carbon dioxide is advantageous since it is harmless to the environment and safe to the human being and has further advantages in that it is easily available anywhere as desired, recycling is not necessary and it is very inexpensive. Therefore, carbon dioxide has heretofore been used as a refrigerant for refrigerators.
With respect to the use of carbon dioxide as the refrigerant, it is disclosed that the sealing property can be improved by using polyvinyl ethers as the refrigerating oil composition for a carbon dioxide refrigerant (Japanese Patent Application Laid-Open No. Heisei 10(1998)-46169).
However, the polyvinyl ethers which are considered to be effective as the refrigerating oil composition for a carbon dioxide refrigerant have a problem due to a small viscosity index in that, when the viscosity is adjusted at a value suitable for surely exhibiting the lubricating property at a high temperature, the viscosity increases at a low temperature and the loss in the power increases.
Therefore, as the lubricant used in combination with the carbon dioxide refrigerant, a refrigerating oil showing a small increase in the viscosity at low temperatures, i.e., a refrigerating oil having a great viscosity index, has been desired.
In the system using the carbon dioxide refrigerant, the refrigerating oil is exposed to carbon dioxide in the supercritical condition since the discharge pressure is higher than that in the system using R-134a or the like and the temperature is higher. Therefore, when a lubricant conventionally used for refrigerants of chlorofluorocarbons, hydrofluoro-carbons or fluorocarbons is used for the lubrication, further problems arise in that stability of the lubrication deteriorates and the stable operation for a long period of time cannot be achieved and that a poor lubricating property such as insufficient wear resistance is exhibited.
The present invention has been made to overcome the above problems. The present invention has an object of providing a refrigerating oil composition exhibiting an excellent lubricating property and a great viscosity index even when a refrigerant containing carbon dioxide as the main component is used as the refrigerant.
The present invention has a further object of providing a refrigerating oil composition exhibiting excellent stability and an excellent lubricating property under the atmosphere of carbon dioxide in the supercritical condition while the above advantageous properties are maintained and can be used for a long period of time.
It was found that, when a polyoxyalkylene glycol was used in combination with a polyvinyl ether, the viscosity index of the polyvinyl ether unexpectedly increased and a refrigerating oil composition exhibiting an excellent lubricating property and a great viscosity index could be obtained and that, when specific additives were used in combination with the refrigerating oil composition, the stability and the lubricating property could be improved even under the atmosphere of carbon dioxide in the supercritical condition while viscosity index and excellent miscibility is maintained. The present invention has been completed based on the above knowledge.
The present invention provides:
(1) A refrigerating oil composition for a carbon dioxide refrigerant which comprises as a main component a mixture comprising (A) a polyvinyl ether having a kinematic viscosity in a range of 3 to 50 mm2/s at 100xc2x0 C. in an amount exceeding 40% by weight and of 99.1% by weight or less and (B) a polyoxyalkylene glycol having a kinematic viscosity in a range of 3 to 50 mm2/s at 100xc2x0 C. in an amount of 0.1% by weight or more and less than 40% by weight;
(2) A refrigerating oil composition for a carbon dioxide refrigerant described in (1), which has a kinematic viscosity in a range of 3 to 50 mm2/s at 100xc2x0 C. and a viscosity index of 90 or greater;
(3) A refrigerating oil composition for a carbon dioxide refrigerant described in (1) or (2), wherein the polyoxyalkylene glycol is a compound represented by general formula (I):
R1xe2x80x94[(OR2)mxe2x80x94OR3]nxe2x80x83xe2x80x83(I)
xe2x80x83wherein R1 represents hydrogen atom, an alkyl group having 1 to 10 carbon atoms, an acyl group having 2 to 10 carbon atoms or an aliphatic hydrocarbon group having 1 to 10 carbon atoms and 2 to 6 bonding portions; R2 represents an alkylene group having 2 to 4 carbon atoms; R3 represents hydrogen atom, an alkyl group having 1 to 10 carbon atoms or an acyl group having 2 to 10 carbon atoms; n represents an integer of 1 to 6; and m represent numbers giving an average value of numbers represented by mxc3x97n in a range of 6 to 80;
(4) A refrigerating oil composition for a carbon dioxide refrigerant described in any of (1) to (3), wherein the polyvinyl ether is a compound represented by general formula (V): 
xe2x80x83wherein R16, R17 and R18 each represents hydrogen atom or a hydrocarbon group having 1 to 8 carbon atoms and the atoms and the groups represented by R16, R17 and R18 may be a same with or different from each other; R19 represents a divalent hydrocarbon group having 1 to 10 carbon atoms; R20 represents a hydrocarbon group having 1 to 20 carbon atoms; k represents numbers giving an average value in a range of 0 to 10; the atoms and the groups represented by R16 to R20 may be a same with or different from each other among constituting units; and, when a plurality of R19O are present, the plurality of R19O may represent a same group or different groups;
(5) A refrigerating oil composition for a carbon dioxide refrigerant described in any of (1) to (4), which comprises an acid scavenger;
(6) A refrigerating oil composition for a carbon dioxide refrigerant described in (5), which comprises 0.005 to 5.0% by weight of the acid scavenger;
(7) A refrigerating oil composition for a carbon dioxide refrigerant described in any of (1) to (6), which comprises an extreme pressure agent;
(8) A refrigerating oil composition for a carbon dioxide refrigerant described in (7), wherein the extreme pressure agent is at least one agent selected from metal salts of carboxylic acids and extreme pressure agents having phosphorus;
(9) A refrigerating oil composition for a carbon dioxide refrigerant described in (7) or (8), which comprises 0.001 to 5.0% by weight of the extreme pressure agent; and
(10) A refrigerating oil composition for a carbon dioxide refrigerant described in any of (1) to (9), wherein the carbon dioxide refrigerant comprises 90% by weight or more of carbon dioxide.